If koi fish can't see obvious pathological changes, doesn't like to eat, swims slowly and becomes emaciated, it is caused by gastrointestinal diseases. You can adjust the pressure difference in the salt bath first, and at the same time, you can take drugs for treating the digestive system, such as gentamicin, or you can take injections in the medicine bath. It is best to take a medicine bath for small fish and an injection for big fish. Another possibility is that your jar is a little small. A friend of mine was moved and gave him some koi fish. After a while, I will see that all those koi fish are kept by him as hairtail. They are long, flat, ugly and unhealthy. Feed is also a problem. Poor feed can also cause gastrointestinal problems in koi fish.